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Moment Coefficient at ACI Code

12/08/2010 10:13 PM

hi Guys!

I am studying about the deflection of one-way, beam, I want to compare the result between apply the moment coefficient and use FEM (use software to modelling), however I have trouble about the moment coefficient, I want to know how to obtain this value (1/11 at support, 1/16 at mid...), the method to get these values.

Re: Moment coefficient at ACI Code.

12/08/2010 11:15 PM

Moment coefficients given in ACI code are approximations. They are on the safe side but they are not theoretically accurate. In order to find accurate moment coefficients, you must consider a set of continuous beams loaded in a variety of ways. If you place live load in alternate spans, you will find the maximum positive moment in the loaded spans. To find maximum negative moment at a particular support, you must place live load on each adjacent span and on alternate spans beyond.

In other words, the maximum moment envelope is developed by considering many different loading patterns. Two cycle moment distribution is a simple and concise hand method of approximating the maximum moments at all points, both positive and negative.

The same is true for software modeling. You must consider all possible load configurations in order to determine the maximum moment at every point. I don't think anybody actually does that because it would be too time consuming. What you can do is consider dead load on every span and calculate the moments from that at every point. Then, consider each span in turn with live load on it. In the end, add up all of the moments which contribute to the effect you are studying and ignore those which do not contribute.

Deflections in any given span will vary according to the loading pattern you select but the maximum deflection in any given span occurs when live load is on that span and also on alternate spans beyond.